Abstract : Denitrification processes are involved in the decrease of nitrate (NO3-) concentrations in groundwater impacted by human activities (André et al., 2011). Non-target/side effects of pesticides on soil microbial denitrification have been demonstrated. It is however now well established that groundwater is an ecosystem in itself with specific microbial communities different from soils’ ones. If the impact of pesticides on groundwater bacterial communities has already been studied (Mauffret et al., 2017), the impact of pesticides on the denitrification activity in groundwater has still to be investigated.
Groundwater samples are collected from several wells on a catchment submitted to agricultural practices with fertilizers and pesticides uses. Groundwaters contain nitrate (lower than limit of quantification up to 80 mg/l), pesticides and some of their metabolites in low concentrations. The abundance of genes involved in denitrification (narG, napA, nirS) shows the occurrence of bacteria able to perform, at least, the two first steps of denitrification. Three groundwaters distributed within the catchment and exhibiting different denitrification signatures based on genes identification and physic-chemical properties are selected. They are used in laboratory to perform batch experiments in conditions boosting microbial denitrification (addition of KNO3 and acetate, anaerobic conditions, 25°C). The microbial ecotoxicology study is conducted in the presence or absence of pesticides spiked as single or cocktail pesticides. Several parameters (pH, concentrations of acetate, NO3, NO2, N2O, biomass) are recorded over time to quantify denitrification rate according to experimental conditions. This experimental work constitutes a first step in the understanding of the inhibitor/catalyzer role of pesticides on denitrification process. In fine, the impact of pesticides on denitrification rate will be included in a simplified numerical model based on the traditional microbially-catalysed denitrification reactions (André et al., 2011).

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Caroline Michel, Nicole Baran, Laurent André, Catherine Joulian. Impact of pesticides on denitrification in groundwaters. 7th edition of the International Conference on Pesticide Behaviour in Soils, Water and Air , Aug 2017, York, United Kingdom. 〈hal-01519859〉